This invention relates generally to systems designed to supply dry pressurized air, and more particularly to systems designed to supply dry pressurized air to underground or outdoor cables, conduit, waveguides, plenums or other air spaces for the purpose of preventing the ingress of moisture or contamination.
Underground air spaces for communications systems, such as cables, conduits, waveguides, and other air plenums, are susceptible to condensation of water vapor from the enclosed air due to variations in temperature and humidity. Typically, a humidity level of significantly less than the ambient dew point is desired in such underground air spaces. To prevent the accumulation of moisture due to elevated humidity levels, the underground air space is normally sealed and pressurized to prevent the ingress or infiltration of moisture-laden ambient air through any small gaps or openings in the confining structure of the air space.
The dry pressurization is currently accomplished in one of several ways. Perhaps, the most common approach utilizes bottles or canisters containing a fixed volume of a pressurized dry gas, such as nitrogen. In this approach, a compressed gas container or bottle is placed above ground adjacent to a manhole that provides underground access to the underground air space. An exposed gas line is extended from the bottle through the manhole access and connected in fluid communication with the underground air space. Dry gas from the bottle provides a positive pressure of a dry pressurized gas that prevents or significantly reduces the ingress of moisture or contamination into the underground airspace.
The use of pressurized bottles has several deficiencies. First and foremost, is the issue of safety. Underground air spaces for communications systems are often located in easements proximate to busy thoroughfares. Gas bottles may be pressurized to internal pressures as great as 3,000 pounds per square inch (psi). Locating such highly pressurized bottles with exposed lines next to busy thoroughfares creates the possibility that a vehicle will strike a gas bottle, causing an explosion, or at least damage the exposed gas line coupling the bottle with the underground air space.
Another significant deficiency of pressurized bottles is that of capacity. Pressurized bottles have a limited capacity for providing continued pressurization based on their size and pressurization. Despite pressurization to significant internal pressures, the supply of dry gas contained in gas bottles of conventional capacity is intermittently depleted. Therefore, the gas bottles must be exchanged periodically to continually provide pressurization.
Another approach to dry pressurization relies on pole-mounted devices that include a dehumidifier or dehydrator to remove moisture from the humid ambient air and a compressor that pressurizes dried air, and the dehumidifying apparatus removes moisture from the pressurized air. The low moisture content of the dried air lowers its dew point so that condensation is unlikely to occur. Among the various types of dehydrators are pressure swing absorption dehydrators and desiccant dehydrators. Although this approach eliminates the dangers associated with pressurized gas bottles, there remain the problems of exposed lines and having to service the device atop the pole in order provided continuous dry pressurization.
Therefore, it would be desirable to have a device that provides dry pressurized air from humid ambient air with lengthened service intervals for pressurizing underground air spaces.